The invention relates to a flat display tube comprising an evacuated envelope which is provided with a display window, a phosphorescent display screen being present on the display window in the evacuated envelope, an emission system for emitting a planar electron beam which initially extends substantially parallel to the display screen, a modulation system for modulating the planar electron beam, which modulation system extends substantially parallel to the display screen, and a deflection system for deflecting the planar electron beam towards the modulation system.
Such a flat display tube is known from U.S. Patent Specification 4,626,899. In the Patent Specification, a description is given of a flat display tube which comprises an electron gun at the side of the display screen, which electron gun emits in operation the planar electron beam in a plane which is substantially parallel to the display screen. This electron gun is provided with a pair of deflection electrodes which extend on either side of the electron beam, and which in cooperation with a planar electrode extending parallel to the modulation system deflect the planar electron beam such that the beam is substantially transversely incident on the modulation system. The location where the planar electron beam is incident on the modulation system is varied by changing the voltages applied to the deflection and planar electrode(s), and the display screen is scanned in a direction transverse to the planar electron beam. The modulation system is controlled by a video signal. By means of this modulation system, the display screen is scanned in a direction transverse to the above-said direction, i.e., in the plane of the planar electron beam.
An important aspect of the picture display quality is the definition. In the known display tube the picture definition in a direction transverse to the planar electron beam is determined by the thickness of the planar electron beam. In order to deflect the planar electron beam in the relatively small space in a flat display tube, preferably, a relatively low-energy electron beam is emitted. By mutual repulsion of the electrons in the planar electron beam, the thickness of the planar electron beam is increased, thereby reducing the picture definition, which is important to, in particular, low-energy electron beams. Low-energy electron beams are to be understood to mean herein, in particular, electron beams comprising electrons having a kinetic energy of less than approximately 3 KeV.